It is a common way that reinforcement members are embedded into rubber products including a rubber belt and a rubber tire, in order to improve the strength and the durability of the rubber products.
Specific examples of the reinforcement members include glass fiber, polyvinyl alcohol fiber as typified by vinylon fiber, polyester fiber, aliphatic polyamide fiber such as nylon and, aromatic polyamide fiber such as polyparaphenylene terephthalamide (hereinafter, called as “aramid”). Among these, the glass fiber and the aramide fiber are widely used.
Since a rubber product such as a rubber belt is repeatedly subjected to bending stress, flexural fatigue easily occurs so as to lower its performance. In addition, delamination between reinforcement fibers and rubber matrix may be caused and the reinforcement fiber may be worn so that the strength of the rubber product is easily lowered. A toothed belt used for driving a cam shaft in an internal combustion engine for automobiles requires high level of dimensional stability in order to provide suitable valve timing. Recently, since the toothed belt is used not only for driving the cam shaft but also for subsidiarily driving an injection pump and the like, high strength and high elastic force are required to withstand high load.
While high-strength glass fiber and aramide fiber have been mainly used as the fiber used for reinforcement of the belt, carbon fiber and polyparaphenylene benzo bis oxazole (hereinafter, abbreviated to “PBO”) fiber have been recently proposed as new materials. Carbon fiber as used in belt tensile body for a toothed belt has been proposed in JPA H08-174708, and PBO fiber has been proposed in JPA H11-336847.
Any of conventional cords for rubber reinforcement made of the aforementioned reinforcement fiber is a cord formed by twisting strands of reinforcement fiber of one kind.
In the conventional cord for rubber reinforcement made of reinforcement fiber of one kind, it is difficult to balance the strength and the flexibility. For example, a cord employing PBO fiber has high tensile strength, but has a problem of being poor in flexibility because of poor compressive fatigue resistance. The PBO fiber can not provide enough adhesion property between the cord and the rubber as matrix. There is a desire for improved adhesion property. A cord employing glass fiber has excellent dimensional stability, but its strength retention is not enough when subjected to bending for a long time.